1. Field of the Invention
The present invention describes a sensing device-Multi-Structure Ion Sensitive Field Effect Transistor and a method to fabricate it from high-pH-sensing membrane of tin oxide (SnO.sub.2) film obtained by thermal evaporation or by r.f. reactive sputtering.
2. Description of the Prior Art
The traditional ion-selecting glass electrode has many advantages, e.g. high linearity, good ion selectivity, high stability. But it has disadvantages such as large volume, high cost and long reaction time. Hon-Sum Wong et al. reported in IEEE Transactions on Electron Devices, Vol. 36 (3), pp. 479-487 (1989) that people tend to replace the traditional ion-selecting glass electrode by an ion-sensitive field effect transistor developed by semiconductor technology.
In IEEE Transactions on Electron Devices, Vol. BME-17(1), pp.59-63 (1970), Piet Bergveld reported that after stripping off the metal gate in a regular metal oxide semiconductor field effect transistor, the device was immersed into aqueous solution. The oxide layer on the gate of the device, as an insulating ion sensing membrane, generates different electrical potentials at the contacting interface in contacting with the solution of different pH values. Thus the channel current varies and the pH value of the solution or the concentration of some other ions can be measured. This phenomenon is referred to by Piet Bergveld as ion-sensitive field effect transistor.
In the 1970s, the development and application of ion-sensitive field effect transistor were still in the investigation stage, as reported by D. Yu et al. in Chemical Sensors, J. Sensor & Transducer Tech., Vol. 1, pp.57-62 (1990). But in the 1980s, the research of ion-sensitive field effect transistor had been raised to a new level. The greatly improved areas were in basic theoretical research, key technology or in the research of practical applications. In other words, more than 20-30 different kinds of field effect transistor transistors, measuring various kinds of ions and chemicals based on the structure of ion-sensitive field effect transistor, have been fabricated. As reported by D. Yu et al. in Chemical Sensors, J. Sensor & Transducer Tech., Vol. 2, pp. 51-55 (1992), ion-sensitive field effect transistors have improved greatly in the areas of minimization, modularizing or multifunctioning. The major reason why the ion-sensitive field effect transistor has become so popular globally is that it has the following special advantages that traditional ion-selecting electrodes lack:
1. Miniaturization so that a minute solution measurement can be performed; PA1 2. High input impedance and low output impedance; PA1 3. Fast response; and PA1 4. The process is compatible with metal-oxide semiconductor field-effect transistor technology.
Since, the ion-sensitive field effect transistor has the above advantages, it has attracted research interest of many researchers over the past twenty years. The more important progress in developing this device internationally during these years is described in the following documents.
W. M. Siu et al. reports with physical and theoretical aspects, IEEE Transactions on Electron Devices, ED-26, Vol. 11, pp. 1805-1815 (1979), that silicon dioxide, silicon nitride, tantalum pentoxide, and aluminum oxide can be the sensing membrane of the ion-sensitive field effect transistor.
A. S. Wong in "Theoretical and Experimental Studies of CVD Aluminum Oxide as a pH Sensitive Dielectric for the Back Contacts ISFET Sensor shows that ion-sensitive field effect transistors with different device structure can use the back contact of the ion-sensitive field effect transistor, or amorphous silicon thin film transistor device for ion-sensitive field effect transistor.
D. Yu, in Chemical Sensors, J. Sensor & Transducer Tech., Vol. 2, pp. 51-55 (1992), reported the miniature of the reference electrode.
B. H. Van Der Schoot et al. reported the integration of measurement system and sensory devices in Sensors and Actuators B, Vol. 4, pp. 239-241 (1991). M. Grattarola et al. reported the simulation research of the ion-sensitive field effect transistor in IEEE Transactions on Electron Devices, Vol. 39 (4), pp. 813-819 (1991).
There are reports of differential type of ion-sensitive field effect transistor. Also, the fixing of enzyme on ion-sensitive field effect transistor to sense the functional signal of biological system, (e.g., sensitive to glucose or sensitive to the oxide concentration in blood, etc.). There is a theoretical study of site-binding model. The research of packaging material and the differential type of ion-sensitive field effect transistor, or the research of packaging material.
U.S. Pat. No. 5,319,226 to Sohn et al. is a method of fabricating an ion sensitive field effect transistor with a Ta.sub.2 O.sub.5 hydrogen ion sensing membrane. In this patent, an r.f. sputtering method is used to fabricate Ta.sub.2 O.sub.5 membrane on the gate area of the ion-sensitive field effect transistor to form Ta.sub.2 O.sub.5 /silicon nitride/silicon dioxide structure of the ion-sensitive field effect transistor.
U.S. Pat. No. 5,407,854 to Baxter et al. is an ESD Protection of ISFET sensors. A method is described for preventing the drifting of electrons in the ion-sensitive field effect transistor.
U.S. Pat. No. 4,609,932 to Anthony describes nonplanar ion sensitive field effect transistor devices. In this patent, the micromachining technology of a laser drill was applied to form a nonplanar structure of the ion-sensitive field effect transistor.
U.S. Pat. No. 4,812,220 to Iida et al. describes an enzyme sensor for determining a concentration of glutamate. The enzyme-type ion-sensitive field effect transistor was used to detect the concentration of amino acid in foods.
U.S. Pat. No. 4,657,658 to Sibbald is for metal oxide semiconductor ion-sensitive field effect transistor devices. The patent describes one metal-oxide-semiconductor field effect transistor and one ion-sensitive field effect transistor which are used to form the differential pair systematic modular system.
According to a report by Tadayuki in Sensors and Actuators B, Vol. 1, pp. 77-96 (1981), for the ion-sensitive field effect transistor on the gate oxide, the most frequently used hydrogen ion sensing membrane are silicon dioxide, silicon nitride, tantalum pentoxide, and aluminum oxide, etc. For silicon nitride and aluminum oxide film, usually it is better to fabricate by low-pressure chemical vapor deposition. Therefore, the processing steps determine the chemical composition of the material, and also determine the properties of this sensing membrane.
Additionally, since the membrane is deposited by low-pressure chemical vapor deposition, the variation of the process condition is much more complicated, e.g. the flow rate ratio of the mixed gas, the process temperature and the pressure of process, etc. Once these conditions vary, the chemical composition of the membrane will be changed, and the characteristics of the sensing membrane will vary too.
For silicon nitride film, if the processing condition is not good enough such that the oxygen concentration in the film is higher, the property of the characteristics of the membrane will get worse. Furthermore, since the price of low-pressure chemical vapor deposition system is more expensive and the gases in the process are quite toxic, so it's not widespread in applications.
According to the report by R. E. G. Van Hal et al. in Sensors and Actuators B, Vol. 24-25, pp. 201-205 (1995), tantalum pentoxide material has the best characteristics for hydrogen ion sensing by r.f. sputtering and the target material of tantalum pentoxide on the gate oxide directly forms the structure of the ion-sensitive field effect transistor. Also, since the target material is used for substrate material directly, the chemical composition of the film is easier to control than those fabricated by low-pressure chemical vapor deposition.
In spite of these difficulties, some international research groups are still making effort to study new types of sensing membrane since this kind of sensing device is quite feasible in practical applications. Those materials that have been studied are: zirconium oxide, titanium oxide, ruthenium oxide, rhodium oxide, iridium oxide, platinum oxide, osmium oxide, etc. But, since the sensing characteristics of these materials is not better than silicon nitride and tantalum pentoxide, etc., they are not widely used in the applications.